This invention relates generally to improvements in capacitor assemblies, particularly of the type used in implantable medical devices such as heart pacemakers and the like to decouple undesired electromagnetic interference (EMI) signals from the device. More specifically, this invention relates to an improved capacitor having a dual element plate configuration for accommodating relatively high pulse currents without degradation or failure. The invention is particularly designed for use in heart pacemakers (bradycardia), defibrillators (tachycardia) and combined pacemaker defibrillator devices.
Ceramic dielectric capacitors are generally known in the art for use in a wide range of electronic circuit applications, for example, for use as charge storage device, a circuit coupling or decoupling device, a filtering device, etc. Such capacitors conventionally comprise a plurality of conductive active and ground electrode plates encased in an alternating stack at a predetermined spacing or gap within a selected dielectric casing material, typically such as a ceramic material formulated to have a selected dielectric constant. The active and ground electrode plates are respectively connected to appropriate conductive termination points or surfaces which facilitate capacitor connection with other elements of an electronic circuit. Exemplary ceramic cased capacitors are shown and described in U.S. Pat. Nos. 4,931,899 and 5,333,095.
In the past, ceramic cased capacitors have been produced by formulating the ceramic casing material into relatively thin sheets. While in a relatively flexible or "green" state before firing, the ceramic sheets are electroded or silkscreened with a refractory metal to define thin conductive plates of selected area. A plurality of these ceramic based sheets with conductive plates thereon are laminated into a stack and then fired to form the sheets into a rigid and dense, substantially monolithic casing structure having the conductive plates embedded therein at a predetermined dielectric spacing.
In operation, the inherent resistance provided by the thin electrode plates results in at least some power loss in the form of plate heating. The plate power loss, and thus the magnitude of plate heating, is a function of electrical current. If the plate current is sufficiently high for even a relatively short period of time, sufficient plate heating can occur to cause capacitor failure, particularly by localized disruption of the thin electrode plates and/or the connections thereof to the conductive termination components. Filter capacitors used in pacemaker and defibrillator applications regularly encounter relatively high pulse inrush currents, and are thus susceptible to overheating and related failures.
One approach to resolving this problem involves increasing the thickness of the electrode plate layers within the capacitor structure. However, a significant increase in capacitor plate thickness is not possible, or desirable or practical through the use of existing electrode plating and silkscreening technologies. Excessively thick electrode plates lead to capacitor plate delamination and related reliability problems. In this regard, it is important for the electrode plates to have a thin and discontinuous structure with ceramic grain growth penetrating through and integrating the entire structure into a rugged monolithic structure. Another approach is to increase the total surface area of the electrode plates, but this concept has required a significant increase in the volumetric size of the capacitor in a manner which is incompatible with many circuit applications.
The present invention overcomes the problems and disadvantages encountered in the prior art by providing an improved ceramic cased capacitor with an embedded electrode plate pattern that is capable of handling significantly higher current loads, without requiring a significant increase in the volumetric capacitor size.